MIMO communication systems come in a variety of different types, including, for example, point-to-point systems and multi-user systems. In a typical point-to-point MIMO communication system, a multi-antenna array in one terminal transmits to a multi-antenna array in another terminal, thereby achieving throughput gains relative to a single-antenna link having similar power and spectral bandwidth. In a typical multi-user MIMO communication system, a multi-antenna array in a base station sends multiple data streams selectively and simultaneously to autonomous single-antenna terminals, again achieving throughput gains relative to a set of single-antenna links. Multi-user systems of this type are sometimes referred to as “broadcast” MIMO systems. The converse to broadcast MIMO is sometimes referred to as “multi-access” MIMO, and it entails the autonomous single-antenna terminals sending multiple data streams simultaneously to the multi-antenna array in the base station.
Multi-user MIMO has a number of advantages relative to point-to-point MIMO. For example, terminals in multi-user MIMO systems can be simple and inexpensive. Also, point-to-point MIMO can fail to deliver the desired high throughput in line-of-sight propagation conditions, while under the same conditions multi-user MIMO continues to provide high throughput as long as the angular separation of the terminals exceeds the Rayleigh resolution of the transmit array. Furthermore, multi-user MIMO seamlessly handles the transition between line-of-sight and rich-scattering propagation.
The principle drawback of multi-user MIMO is that the base station has to know the propagation characteristics of the forward channel. The process through which the base station obtains this information is generally referred to as training. See, for example, U.S. Patent Application Publication No. 2005/0265290 to Hochwald et al. entitled “Feedback Method For Channel State Information of a Wireless Link,” which is commonly assigned herewith and incorporated by reference herein.
It is recognized that the acquisition of such forward channel information is facilitated by time-division duplex (TDD) operation. In the TDD context, the principle of reciprocity implies that the reverse channel matrix is equal to the transpose of the forward channel matrix, so the base station can readily obtain the required forward channel information by processing pilot signals transmitted by the terminals on the reverse link.
Unfortunately, conventional approaches to multi-user MIMO communication may not be optimally configured for high terminal mobility situations. For example, under propagation conditions associated with high mobility, conventional approaches may not allow enough time before the channel changes to transmit reverse pilots and then transmit the forward data streams. Also, under low signal to interference-plus-noise ratio (SINR) conditions, an excessive number of pilot symbols may be required to obtain a channel estimate of sufficiently good quality to enable selective transmission of the data streams. Thus, systems based on these conventional approaches may be unable to obtain the full throughput advantages of multi-user MIMO communication.
A need therefore exists for improved approaches to multi-user MIMO communication, particularly in terms of providing improved system throughput in the presence of high terminal mobility.